Pablo,
nova compute logs are missing. According to the customer case:
"""
But, in /var/log/nova/nova-compute.log we can see the following error:
2016-03-08 10:42:02.157 31018 TRACE nova.compute.manager [instance: ac1c7a41-fb72-4918-ad4f-f7411f71aacc] InvalidInput: Invalid input received: Invalid volume: being attached by another instance
"""
Which means something happened on Mar 08, 2016 but since compute log is completely missing and cinder volume log from March 08 is also missing its impossible to understand what happened.
It looks like attach has failed somewhere between Nova and Cinder but hard to say something more specific.
Please, attach a missing logs or ask the customer to reproduce the case and reopen that BZ.